1. Field of the Invention
This invention relates to a polyester film and a magnetic recording medium comprising it as a base. Particularly, it relates to a polyester film whose surface has excellent flatness, slipperiness and adhesion and a magnetic recording medium comprising the polyester film as a base.
2. Description of the Prior Art
Recently, polyester films have markedly been developed in the fields for applications to magnetic recording media such as magnetic tapes and disks, and to capacitors, photographs, microfilms, packagings and the like.
In these applications of polyester films, strictly controlled surface properties are required For example, in the applications to magnetic recording media such as magnetic tapes, a base film must have, at the same time, excellent flatness for good recording and playback qualities (herein referred to as "electromagnetic conversion property"), excellent slipperiness for good head contact and running property, and excellent adhesive property for good adhesion of a magnetic layer in order to maintain good electromagnetic conversion and running properties. However, conventional polyester films have hardly satisfied all the three properties, that is, flatness, slipperiness and adhesion, simultaneously.
There have been proposed polyester films containing internal particles precipitated in the polyester-forming reaction system and inert particles incorporated after polymerization as disclosed in U.S. Pat. Nos. 4,096,109 and 4,138,386, a laminated film of which both sides have different properties, respectively, as disclosed in U.S. Pat. No. 3,515,626, and a biaxially stretched polyester film with a coating as disclosed in U.S. Pat. No. 4,364,885.
However, a film surface having simultaneously flatness and slipperiness could not have been obtained in the prior art. Furthermore, such laminated films and coated films as described above, which have one surface with flatness and easily adhesive property and another surface with good slipperiness formed by lamination or coating, had such a transcription defect between one surface and another so that the properties of each surface could not be maintained. Briefly, a film having all the three properties, that is, flatness, slipperiness and adhesion, have not been obtained in the prior art, in other words, a magnetic tape having both good running and electromagnetic conversion properties could not have been obtained from those films of the prior art.